1. Field of the Invention
The present invention relates to an apparatus for measurement of a polarized distribution, a polarizing filter for using in the apparatus and a polarizing filter assembly.
2. Description of the Prior Art
In the field of medical ophthalmology, glaucoma is known as a disease resulting from excavation or deficiency in the nerve fiber layers as caused by increased intraocular pressure, among others. At an advanced stage, glaucoma causes irreversible vision disorder, leading to loss of sight. Therefore, early discovery of signs of glaucoma is regarded as an urgent task. However, the nerve fiber layers vary in thickness from portion to portion on an ocular fundus and the progress of the defect of the nerve fiber layers is generally gradual until the morbid condition becomes severe, so that even when the fundus image is available as a result of ophthalmography of the fundus of a patient at an early symptom stage, it is difficult for an oculist to judge, based on that image, whether there is a defective area in the nerve fiber layers.
Meanwhile, it is known that the nerve fiber layers are a birefringent substance showing different refractive indices according to the direction of vibration (polarization) of light.
Thus, when the nerve fiber layers are irradiated with a certain polarized laser beam, the polarized light components pass through that layer at different velocities, resulting in different times for passing through the nerve fiber layer for each of the components of the polarized laser beam.
The magnitude of this phase difference is positively correlated with the thickness of the nerve fiber layers at the portion through which the light has passed.
The polarized light is classified simply into linearly polarized light, circularly polarized light and elliptically polarized light according to the quantity of phase difference between the polarized light components.
Utilizing this fact, a glaucoma diagnosis apparatus has been developed which converts a laser beam from a laser diode to the linearly polarized state by means of a polarizing filter having linearly polarizing characteristics, modulates this laser beam to circularly polarized light using a quarter wavelength plate, deflects that laser beam by means of a scanning unit to scan the ocular fundus therewith, detects the rate of change in phase difference of the light reflected from the ocular fundus and determines the thickness of the nerve fiber layers.
However, this glaucoma diagnosis apparatus in the prior art can only scan a certain narrow area of the ocular fundus and cannot examine a wide area at a time. It has another problem that the data cannot be obtained as image information.
A further problem is that such a glaucoma diagnosis apparatus tends to be expensive and is still hardly available for ordinary ophthalmologists.
Further, hitherto there is no an apparatus for measuring distribution of a polarized light for a subject to be examined.
Accordingly, it is an object of the present invention, which has been made in view of the limitations of the related art discussed above, to provide a glaucoma diagnosis apparatus capable of determining the thickness of the nerve fiber layers in a simple, easy and inexpensive manner by improving the optical system of a currently existing retinal or fundus camera in common use.
It is another object of the present invention to provide an apparatus for effective measurement of a polarized distribution of a subject to be examined.
To accomplish the above object, an apparatus for measurement of a polarized distribution according to the present invention comprises: an illuminating optical system for illuminating a subject to be examined by circularly polarized detecting light; and an image receiving optical system for receiving detecting light from the subject which is illuminated by the circularly polarized detecting light.
The image receiving optical system includes a CCD element having an imaging plane which is provided integrally with a polarizing filter.
In one embodiment, the polarizing filter is composed of repetitions of a set of unit platelets each comprising polarizing micro-plates for decomposing the detecting light into first linearly polarized light components which are mutually perpendicular and polarizing micro-plates for decomposing the detecting light into second linearly polarized light components which are in a direction of crossing with the first linearly polarized light components and which are mutually perpendicular.
In other embodiment, the polarizing filter is composed of repetitions of a set of unit platelets comprising polarizing micro-plates for decomposing circularly polarized light into first linearly polarized light components which are mutually perpendicular and polarizing micro-plates for decomposing said circularly polarized light into second linearly polarized light components in a direction of closing with said first linearly polarized light components.
According to the present invention, a polarizing filter assembly is provided. The assembly comprises a polarized filter and a CCD element formed integrally with the polarized filter. The polarized filter is composed of repetitions of a set of unit platelets comprising polarizing micro-plates for decomposing polarized light into first linearly polarized light components which are mutually perpendicular and polarizing micro-plates for decomposing said polarized light into second linearly polarized light components in a direction of closing with said first linearly polarized light components.
The CCD element receives light passing through said polarized filter.
In one embodiment, the polarizing filter is formed in such a manner that the directions of the second linearly polarized light components which are crossing with both of the first linearly polarized light components are at an angle of 45xc2x0 with respect to both of the first linearly polarized light components.
In one embodiment, each polarizing micro-plate corresponds to one pixel of said CCD element.